Draws a document that was READ — parsed from source, or decoded off
the editor's wire. There is no definition behind one of those, which is
why it is a door of its own rather than the same one.
What animates this scene, bound to it. Mounting one REGISTERS A
PLAYHEAD: an export walks it, a test parks it, and previews can
photograph the scene at any moment without the app wiring anything.
How a registered kind (SceneKind) is drawn, by the kind's name — the
door a renderer the core never imports comes through, the way an
external widget's builder does. A kind with no renderer draws a named
placeholder, never a silent gap.
The live document. Edits and fx writes both notify it, and this widget
redraws from the RENDERED plane — base composed with every fx writer —
so a playing motion and a saved layout are one picture.
